The present invention relates to a demodulator for demodulating an amplitude-modulated carrier signal, such as a carrier signal modulated by amplitude-shift keying.
Amplitude-shift keying (abbreviated ASK) is a modulation system employed in various wireless systems, including certain digital mobile communication systems. A binary ASK transmitter, for example, transmits a radio-frequency (RF) carrier signal having a large amplitude for one binary value (e.g. `1`), and a small amplitude for the other binary value (e.g. `0`). Alternatively, the carrier signal is transmitted for one value and not transmitted for the other value.
A receiver equipped with a conventional ASK demodulator receives the RF carrier signal, down-converts the received signal to an intermediate frequency (IF), performs automatic gain control (AGC), then detects the envelope of the IF signal to produce a baseband signal, and decides from the amplitude of the baseband signal whether each transmitted bit of information is a one or a zero.
The high speed of recent digital communication systems, however, requires an AGC loop with fast tracking capability, in order to maintain the proper levels in the baseband signal. This, in turn, requires complex and expensive AGC circuitry.
Moreover, mobile systems in particular often transmit data in bursts. If the time constant of the AGC loop is set long enough to maintain steady `1` and `2` levels, there is a period at the beginning of each burst in which the AGC loop is unsettled and data errors become likely. If the time constant of the AGC loop is shortened to avoid this situation, the danger exists that the AGC loop will oscillate in response to a pattern of 1s and 0s.